The present invention relates, in general, to a catalyst for selective catalytic reduction of nitrogen oxides using spent catalyst and a method for preparing the same. More specifically, the present invention relates to a catalyst prepared by use of raw materials unsuitable for general catalysts for selective catalytic reduction of nitrogen oxides, for example spent catalyst mainly adsorbed with vanadium, nickel and sulfur in the hydro-desulfurization line of an oil refinery, in which a catalyst for the hydro-desulfurization contains molybdenum, iron, cobalt, silicon and so on, and a method for preparing the same.
Generally, exhaust gas discharged during combustion of hydrocarbon fuels such as hydrocarbon gas, gasoline or diesel fuel may cause serious pollutions of the environment. Various pollutants present in such exhaust gas are compounds containing hydrocarbon and oxygen, in particular, nitrogen oxides (NOx) and carbon monoxide (CO). Therefore, extensive and intensive research has been carried out for years to decrease the amount of harmful gas exhausted from coal-steam power plants, incinerators or combustion systems in the automobile industry.
Techniques for effectively eliminating nitrogen oxides (NOx) are commonly classified into a selective catalytic reduction (SCR) using a catalyst and a reductant together, a selective non-catalytic reduction (SNCR) using only a reductant without a catalyst, a low-NOx burner controlling a combustion state in the burner and so on.
Among the aforementioned techniques, the selective catalytic reduction is valued as the most effective technique for removing nitrogen oxides, taking account of the generation of secondary pollution, removal efficiency, operation cost, etc. In case of using the selective catalytic reduction technique, nitrogen oxides may be removed with an efficiency of 90% or greater and the endurance period of the catalyst is about 2 to 5 years. In addition, SCR technique is technically advantageous because poisonous dioxin may be removed, along with nitrogen oxides, in the incinerator.
Catalysts useful in the selective catalytic reduction are classified into an extrusion catalyst, a metal plate catalyst, and a pellet catalyst, depending on their external forms. Currently, the extrusion catalyst and the metal plate catalyst are widely used in steam plants and incinerators, and the pellet catalyst is suitable for the preparation processes of nitric acid, caprolactam, etc. The catalyst composition mainly comprises oxides of active metals such as vanadium, molybdenum, tungsten and the like, and further comprises other transition metal components for shifting active temperature ranges and enhancing durability of the catalyst.
Useful as a support in the selective catalytic reduction techniques is titania (TiO2), alumina (Al2O3), silica (SiO2) and so on, in which titania is more preferably used in terms of the poisoning resistance thereof.
In the selective catalytic reduction technique, a catalyst manufactured by impregnating said support with catalytic components may be coated on the structural bodies such as the metal plate or ceramic honeycomb to prepare a catalytic body, through which exhaust gas can be passed, whereby harmful gases such as nitrogen oxides are adsorbed and thus-reduced into harmless materials.
In this regard, U.S. Pat. No. 5,827,489 discloses a process for the preparation of a catalyst for selective catalytic reduction containing oxides of crystal phases by impregnating a support of inorganic oxides such as titania, alumina, silica and zirconia with catalytic components such as vanadium, molybdenum, nickel and tungsten, thereafter heat treating. This patent employs a support and catalytic components with a superior poisoning resistance versus sulfur oxides for the selective catalytic reduction and has advantages of freely controlling the amounts of active metals, a specific surface area and pore sizes of the catalyst to prepare the catalyst having optimal performance in which a suitable amount of sulfate is added. On the other hand, it suffers from high preparation cost because, each of single materials (or precursors) used as the support and the catalyst should be prepared by methods of catalyst production and mixing.
Meanwhile, Korean Patent Laid-Open No. 95-72277 refers to recycling of spent catalysts discharged from a hydro-desulfurization process of an oil refinery. A catalyst for selective catalytic reduction prepared by recycling such spent catalysts in the presence of ammonia as a reducing agent according to said invention is more advantageous in terms of low preparation cost, inherent poisoning resistance versus sulfur oxides, and containing the high content of metal components with excellent activities for nitrogen oxides reduction, compared with a catalyst prepared by a combination process of single materials.
However, the above-mentioned invention suffers from the disadvantage that the spent catalyst discharged from different discharge-lines after a desulfurization process has different properties, thereby lowering catalytic performance. In case of using spent catalyst for selective catalytic reduction, it may be recycled with only a 30% recovery rate on the basis of the whole discharged amounts of the spent catalyst, and thus intensive research for solving said problems has been carried out.
Leading to the present invention, the intensive and thorough research on spent catalyst discharged from a hydro-desulfurization process of an oil refinery, carried out by the present inventors aiming to avoid the problems encountered in the prior arts, resulted in the finding that unsuitable raw materials as a catalyst for selective catalytic reduction of nitrogen oxides may be recycled to prepare a catalyst for selective catalytic reduction of nitrogen oxides which has excellent removal efficiency of nitrogen oxides.
Therefore, it is an object of the present invention to provide a catalyst for selective catalytic reduction of nitrogen oxides by use of spent catalyst, which can increase an efficiency of spent catalyst reclamation and has excellent removal effect of nitrogen oxides.
It is another object of the present invention to provide a method for preparing a catalyst for selective catalytic reduction of nitrogen oxides using spent catalyst.
To achieve said objects of the present invention, there is provided a method for preparing a catalyst for selective catalytic reduction of nitrogen oxides from spent catalysts having been adsorbed with vanadium, nickel and sulfur in the hydro-desulfurization line of an oil refinery in which a catalyst for the hydro-desulfurization contains molybdenum, iron, cobalt and silicon on the alumina support, comprising the following steps of:
providing a first material comprising 10 wt % or more of vanadium, 5 wt % or more of nickel, 5 wt % or more of sulfur and 3 wt % or less of molybdenum and having a specific surface area of 60 m2/g or smaller and pore sizes of 250 xc3x85 or larger from said spent catalysts;
providing a second material comprising 3 wt % or less of vanadium, 4 wt % or less of nickel, 2 wt % or less of sulfur and 5 wt % or more of molybdenum and having a specific surface area of 130 m2/g or larger and pore sizes of 150 xc3x85 or smaller from said spent catalysts;
pretreating the first and second materials, separately, at 300 to 400xc2x0 C.; pulverizing each of the pretreated materials to an average particle size of 100 to 200 mesh;
mixing the first material with acid and water, then with the second material; and
drying the mixture at 100 to 120xc2x0 C. and then calcining at 450 to 550xc2x0 C.